Journal of Ocean University of China  2020, Vol. 19 Issue (5): 1051-1061  DOI: 10.1007/s11802-020-4231-9

Citation  

XU Yue, YANG Yaoming, YU Hongjun, et al. Geochemistry and Petrogenesis of Volcanic Rocks from the Continent-Ocean Transition Zone in Northern South China Sea and Their Tectonic Implications[J]. Journal of Ocean University of China, 2020, 19(5): 1051-1061.

Corresponding author

YU Hongjun, Tel: 0086-532-67726668 E-mail: hjyu@ndsc.org.cn.

History

Received May 9, 2019
revised November 21, 2019
accepted January 13, 2020
Contents              Abstract              Full text              Figures/Tables              PDF
Geochemistry and Petrogenesis of Volcanic Rocks from the Continent-Ocean Transition Zone in Northern South China Sea and Their Tectonic Implications
XU Yue1),2),3) , YANG Yaoming2) , YU Hongjun1),2) , GAO Wei2) , GAO Xiangxing2) , LIU Baohua1),2) , TIAN Xu2) , YANG Jichao2) , and ZHANG Wenquan2)     
1) Deep Sea Interdisciplinary Center, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266061, China;
2) National Deep Sea Center, Ministry of Natural Resources, Qingdao 266237, China;
3) College of Marine Geosciences, Ocean University of China, Qingdao 266100, China
Received May 9, 2019; revised November 21, 2019; accepted January 13, 2020
Corresponding author: YU Hongjun, Tel: 0086-532-67726668 E-mail: hjyu@ndsc.org.cn.
Abstract: Miocene–Pliocene (22–5 Myr) volcanism and associated seamounts are abundant in the continent-ocean transition (COT) zone in the margin of the north South China Sea (SCS). The petrogenesis of volcanic rocks from these seamounts and regional tectonic evolution of COT zone are poorly known. In this paper, we obtained whole-rock major and trace element compositions and Sr-Nd-Pb isotopic data for these volcanic rocks from the Puyuan and Beipo seamounts within COT zone, in northeastern SCS. Based on the geochemical analyses, the volcanic rocks are classified as alkaline ocean island basalts (OIB) and enriched mid-ocean ridge basalts (E-MORB). The OIBs from the Puyuan seamount are alkaline trachybasalts and tephrites that show enrichment of the light rare earth elements (LREE) relative to heavy rare earth elements (HREE) and more radiogenic Sr-Nd isotopic compositions, and have significant 'Dupal isotopic anomaly'. In contrast, the E-MORBs from the Beipo seamount are tholeiitic basalts that have less enrichment in LREE and less radiogenic Sr-Nd isotopic compositions than the counterparts from the Puyuan seamount. Petrological and geochemical differences between the OIBs and MORBs from these two seamounts can be explained by different mantle sources and tectonic evolution stages of the COT zone. Syn-spreading OIB type basalts from the Puyuan seamount were derived from an isotopically 'enriched', and garnet facies-dominated pyroxenitic mantle transferred by the Hainan mantle plume. In contrast, post-spreading E-MORB type basalts from the Beipo seamount are considered to be derived from the melting of isotopically 'depleted' pyroxenite mantle triggered by lithosphere bending and subsequent post-rifting at the lower continental slope of the northern margin.
Key words: volcanic rocks    geochemistry    tectonic evolution    South China Sea    continent-ocean transition zone    
1 Introduction

The continent-ocean transition (COT) zone of the passive margin in the north South China Sea (SCS) is a significant transition region between continental and oceanic crust, and the rocks occurred records the SCS extension and subsequent rifting process (Lester et al., 2014; Eagles et al., 2015; Sibuet et al., 2016; Zhao et al., 2018). The spreading and rifting in COT has been accompanied with a series of magmatic activities and associated seamounts that were identified in seismic profiles (Briais et al., 1993; Lüdmann and Wong, 1999; Zhu et al., 2012). Most previous studies focused on volcanic rocks from subbasin seamounts and Hainan-Leizhou-Indochina area, and these rocks have been used to investigate the composition of the mantle sources and the geodynamic processes in SCS sub-basin and adjacent areas (e.g., Hoang and Flower, 1998; Zou and Fan, 2010; Wang et al., 2012b, 2013; Hoang et al., 2013; Yan et al., 2014, 2015; Zhang et al., 2017, 2018). However, little has been reported that deals mainly with volcanic seamounts in the COT zone, as it is difficult to collect samples from these seamounts. Questions concerning the nature of the mantle source and the tectonic evolution in the COT zone are unresolved (Zhu et al., 2012; Wu et al., 2014; Gao et al., 2015, 2016). Answers may be found in the geochemistry of volcanic rocks from the seamounts within the COT zone.

In this study, we present new whole-rock elemental and Sr-Nd-Pb isotopic compositions of volcanic rocks from Puyuan and Beipo seamounts that were acquired by two dives of the submersible Jiaolong within COT zone. We compare the geochemical characteristics between syn-spreading OIB type basalts from Puyuan seamount and postspreading E-MORB type basalts from Beipo seamount, and draw conclusions on the petrogenesis of the two type volcanic rocks and their mantle sources at different evolutional stages, and propose a model in which different tectonic evolution scenario are responsible for the heterogeneity.

2 Geological Background and Sample Description 2.1 Geological Background

The SCS is a Cenozoic marginal sea and formed by the interactions among the Eurasia plate, the Indian plate, the Australian plate and the Pacific plate (Sun et al., 2006; Xia et al., 2006). Based on bathymetry and magnetic data, the seafloor spreading of SCS basin occurred from 33 to 15 Myr in a roughly NS direction (Taylor and Hayes, 1980, 1983; Briais et al., 1993; Li et al., 2014). The seafloor spreading in the east sub-basin initiated at about 33 Myr and terminated at about 15 Myr, whereas the spreading of southwest sub-basin started at about 23.6 Myr and terminated at about 16 Myr (Li et al., 2014).

A hypothesized young plume, termed as 'Hainan mantle plume', located near Hainan Island-Leizhou Peninsula was observed by seismic tomography (e.g., Lebedev and Nolet, 2003; Huang and Zhao, 2006; Zhao, 2007; Lei et al., 2009). The plume-like mantle is further confirmed by extensive Miocene – Pliocene OIB-type basalts, including continental flood basalts (17–1 Myr) in the Hainan-Leizhou-Indochina area and fossil ridge basalts (15.7–3.5 Myr) in the eastern and southwestern sub-basin of SCS (Fig.1; e.g., Tu et al., 1991; Hoang and Flower, 1998; Zou and Fan, 2010; Wang et al., 2012b, 2013; Yan et al., 2015; Zhang et al., 2018).

Fig. 1 (a) Simplified geological map of the COT zone of SCS and the location of Puyuan and Beipo seamounts, Dongsha Rise, Manila Trench, and the boundary of COT zone. Insert shows the spatial distribution of SCS, Indo-China Peninsula, Hainan-Leizhou Peninsula and fossil ridge seamount; (b) 3D plot of Beipo seamount; (c) 3D plot of Puyuan seamount. Bathymetric data are from http://www.geomapapp.org/.

The COT zone in the margin of the north SCS is a region of transition between oceanic crust and continental crust with normal thickness during SCS spreading (Mjelde et al., 2007; Minshull, 2009; Franke et al., 2011). The initial opening of the northern continental slope may have started at around 37 Myr, earlier than the sub-basin (Hsu et al., 2004).

The Puyuan seamount (22–21 Myr) marked an episode of sys-spreading intra-plate volcanism in the northeastern COT zone (Fig.1; Wang et al., 2012a). After spreading cessation (< 15 Myr), continental thinning of COT zone has been accommodated by a series of rises (e.g., Dongsha Rise) and depressions (McIntosh et al., 2013; Wu et al., 2014). Based on seismic data, several late Miocene – Pliocene volcanic centers in the Dongsha Rise and adjacent depressions, including the Beipo seamount, have been identified within the COT zone (Fig.1; Lüdmann and Wong, 1999; Zhu et al., 2012). Recent wide-angle tomographic velocity models have shown that continental crust in the COT zone has subducted at the Manila Trench underneath the Philippine plate after the late Miocene (McIntosh et al., 2013).

2.2 Sample Description

Two dives (Dives 139 and 141) have been performed in the COT zone of the northeastern SCS margin by submersible Jiaolong, R/V Xiangyanghong 09 Dayang 38th-Ⅱ cruise during May 5–8, 2017. Dive 139 was carried out across the Puyuan seamount, and Dive 141 was performed on the Beipo seamount.

Six fresh rock samples (Dive 139) were collected from Puyuan seamount, including trachybasalts and tephrites (Figs. 2a–c). Trachybasalts are porphyritic with phenocrysts dominated by plagioclase and clinopyroxene in a groundmass of plagioclase-sanidine-Fe-Ti oxide (Figs. 2e and f; Table 1). Tephrites contain phenocrysts of clinopyroxene, sanidine and plagioclase. Samples are fresh to moderately altered with carbonate infillings (Fig.2g).

Fig. 2 Hand specimens and photomicrographs of volcanic rocks from Puyuan (a–c, e–g) and Beipo (d, h) seamounts. Cpx, clinopyroxene; Opx, orthopyroxene; Pl, plagioclase; Sa, sanidine.
Table 1 Phenocryst and groundmass mineralogy of the basalts from the Puyuan and Beipo seamounts

Two basalts (Dive 141) from Beipo seamount contain phenocrysts of clinopyroxene, sanidine, plagioclase, and minor orthopyroxene (Figs. 2d and h; Table 1). Minerals in the hyalopilitic groundmass are mainly composed of microlitic clinopyroxene, sanidine, and plagioclase.

3 Analytical Methods 3.1 Whole-Rock Major- and Trace-Element Analyses

Rock samples were cut into thin slabs and the freshest portions were used for bulk-rock analyses. Major elemental compositions were determined by an X-ray fluorescence spectrometer (PANalytical Axios-advance) at the ALS Laboratory Group, Guangzhou. 1 g sample powder was heated up to 1100℃ for 1 h to calculate loss on ignition (LOI). Analytical precision was generally better than 5%, as determined based on the Chinese National standard GSR-3.

Trace elements were measured by using a PE DRC-e ICPMS at the state Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences. Rock powder (50 mg) was dissolved in PTFE lined stainless steel bombs using a mixture of HF and HNO3 for 48 h at 190℃. As an internal standard, Rh was used to monitor signal drift during counting. The international standards JG-2, SG-3, GSR-1, G-2, NIM-G, and SG-1a were used for monitoring analytical quality. The analytical uncertainty is lower than 5%. The detailed analytical methods were described by Qi et al. (2000).

3.2 Whole-Rock Sr-Nd-Pb Isotope Analyses

The Sr-Nd-Pb isotopic compositions were analyzed by using on a Finnigan Neptune plus MC-ICP-MS at Institute of Oceanology, Chinese Academy of Sciences. Wholerock powders were dissolved in Teflon bombs using HF + HNO3. Sr and Nd were separated in solution by using cationic ion-exchange procedures (Richard et al., 1976; Zhang et al., 2002; Fan et al., 2003). Sr and Nd isotopic ratios were normalized to 86Sr/88Sr of 0.1194 and 146Nd/ 144Nd of 0.72419. Standard Reference material NBS987 for Sr yielded average 87Sr/86Sr = 0.710216 ± 0.000012 (2σ, n = 10) and Standard Reference material Jndi-1 ESA Delf for Nd yielded 143Nd/144Nd = 0.512118 ± 0.000006 (2σ, n = 10).

Lead was separated and purified using conventional anion-exchange techniques with dilute HBr as eluent (Zhang et al., 2002). The whole procedural blank was less than 0.4 ng Pb. During the analyses, the international standard material NBS981 yielded a weighted mean 206Pb/204Pb of 16.935 ± 0.0004 (2σ, n = 8) (the recommended value is 16.9356), a 207Pb/204Pb of 15.486 ± 0.0004 (2σ, n = 8) (the recommended value is 15.4891), and a 208Pb/204Pb of 36.691 ± 0.010 (2σ, n = 8) (the recommended value is 36.7006) (Todt et al., 1996).

4 Results 4.1 Major and Trace Elements

Major and trace elemental composition results of Puyuan and Beipo basalts are presented in Table 2. The Puyuan volcanic rocks vary from tephrite to trachybasalt (SiO2 = (41.94 – 44.96) wt%; K2O = (1.15 – 1.34) wt%; Na2O = (2.88 –3.00) wt%) with Mg-numbers (Mg#) ranging from 38 to 47 [Mg# = molar Mg×100/(Mg + Fe2+)] and high TFe2O3 contents ((11.36 – 12.40) wt%) (Table 2). They have high K2O, Na2O contents and Nb/Y ratios, and belong to alkaline series (Fig.3). The tephrites have higher CaO contents than that of trachybasalts. In terms of trace elements, they show enrichment in the LREE and LILE relative to HREE and HFSE with steep chondrite-normalized REE patterns [(La/Yb)N = 6.75-16.27, (La/Sm)N = 3.31-5.26, (Er/Yb)N = 1.16-1.51] and absence of Eu negative anomalies (Eu/Eu* = 0.89 – 1.06), which exhibit OIB affinity (Fig.4; Table 2).

Table 2 Major (wt%) and trace elemental (×10−6) concentrations of basalts from Puyuan and Beipo seamounts
Fig. 3 Geochemical classification of volcanic rocks from Puyuan (a–c) and Beipo (d) seamounts. (a) TAS diagram (Le Maitre et al., 1989); the dashed line separating alkaline series from subalkaline series is from Irvine and Baragar (1971); all the major element values are normalized to 100% on a volatile-free basis. (b) Nb/Y vs. Zr/(P2O5×104) (Floyd and Winchester, 1975).
Fig. 4 (a) Primitive mantle normalized multi-element and (b) chondrite-normalized rare earth element (REE) pattern diagrams for volcanic rocks from Puyuan and Beipo seamounts. Chondrite and primitive mantle normalizing values, ocean island basalt (OIB), enriched mid-ocean ridge basalt (E-MORB) and normal mid-ocean ridge basalt (N-MORB) are from Sun and McDonough (1989).

The basalts from Beipo seamount have high SiO2 ((49.32 – 49.41) wt%) and Na2O ((3.12 – 3.15) wt%) but low K2O ((0.78 – 0.81) wt%) contents, and plot in the tholeiitic basalt field on the Zr/(P2O5×104)-Nb/Y diagram (Fig.3b). Compared with the alkaline ones from Puyuan seamount, they have similar MgO ((4.10 – 4.14) wt%) but lower CaO ((9.75 – 9.81) wt%) and TFe2O3 ((10.79 – 10.84) wt%) contents. They exhibit LREE enrichments with relatively flat REE patterns [(La/Yb)N = 2.37-3.15, (La/Sm)N = 1.46-1.69, (Er/Yb)N = 1.03-1.31], and are similar to the averaged E-MORB defined by Sun and McDonough (1989) (Fig.4; Table 1).

4.2 Sr-Nd-Pb Isotopes

Sr, Nd, and Pb isotopic compositions are presented in Table 3, and plotted in Fig.5. The Puyuan alkaline basalts have 87Sr/86Sr ratios of 0.704360 – 0.705146 and εNd of 2.0 – 5.3 (Fig.5a). They have narrow ranges of 206Pb/204Pb (18.6463 – 18.6822), 207Pb/204Pb (15.6408 – 15.6560), and 208Pb/204Pb (38.8670 – 38.9150), plotting well in the field of 'Dupal isotopic anomaly' (Fig.5b). The Beipo tholeiitic basalts show almost the same Pb isotopic compositions (206Pb/204Pb = 18.6345, 207Pb/204Pb = 15.6579, and 208Pb/204Pb = 38.8917) as those of the Puyuan basalts. However, they have lower 87Sr/86Sr ratios of 0.703680 and higher εNd of 6.6 than those of Puyuan basalts (Fig.5).

Table 3 Sr-Nd-Pb isotopic data for the basalts from Puyuan and Beipo seamounts
Fig. 5 (a) εNd vs. 87Sr/86Sr; (b) 207Pb/204Pb vs. 206Pb/204Pb; (c) 208Pb/204Pb vs. 206Pb/204Pb; (d) 87Sr/86Sr vs. 206Pb/204Pb. The NHRL (Northern Hemisphere Reference Line; Hart, 1984), enriched mantle Ⅰ (EMI; Zindler and Hart, 1986), enriched mantle Ⅱ (EMII; Hart et al., 1992) and Dupal Pb anomaly (Hamelin and Allègre, 1985) are shown for reference. The aubergine dot-dash lines are mixing curves between depleted mantle (DM; Workman and Hart, 2005; 87Sr/86Sr = 0.7026, Sr = 7.66×10−6, εNd = 9.3, and Nd = 0.58×10−6) and enriched mantle Ⅱ (EMII; Hart et al., 1992; 87Sr/86Sr = 0.7078, εNd = −1); numbers beside the mixing curves indicate the weight fraction of EMII end member in the mixture. Isotope data for the East Pacific Rise (EPR) and the Indian Ocean MORBs (Gale et al., 2013), Hainan OIBs (Han et al., 2009; Zou and Fan, 2010; Li et al., 2013), Indochina OIBs (Hoang et al., 1996, 2013), SCS fossil ridge seamount (Tu et al., 1992; Yan et al., 2008; Expedition 349 Scientists, 2014) were plotted for comparison.
5 Discussion 5.1 Petrogenesis of Volcanic Rocks from Puyuan Seamount and Beipo Seamount

In view of high loss on ignition (LOI) of some samples (Table 2), secondary seawater alteration has to be considered. CaO and LILEs (e.g., K, Sr, Ba, Cs and Rb) are generally mobile and tend to display decreasing trends with increasing LOI contents in seawater altered rocks (Smith and Smith, 1976; Bédard, 1999; Wang et al., 2006). However, none of these trends have been observed for the studied basaltic samples (Fig.6). Thus, the effect of secondary seawater alteration appears to be negligible in the study of petrogenesis for the basalts from the Puyuan and Beipo seamounts in this paper.

Fig. 6 Plots of incompatible elements (K2O, CaO, Rb, Sr, Cs and Ba) vs. LOI contents.

We assess the effect of crustal contamination during the generation of these basalts. Generally, their low SiO2 and high MgO contents may reflect their mantle origin and limited degree of crustal contamination (Yan et al., 2015; Tian et al., 2019). Furthermore, large crustal contamination can be ruled out based on the following observations: 1) all these rocks have relatively uniform Ni and Cr contents (Table 2); 2) their 206Pb/204Pb ratios show restricted range (Fig.5d), as crustal contamination tend to result in a large variation in 206Pb/204Pb (Jiang et al., 2006); 3) there are no crust-derived xenocrysts in these rocks (Fig.2). Therefore, their geochemical compositions could be used to reflect the nature of mantle source region.

5.1.1 OIB type basalts from Puyuan seamount

Direct partial melting of the mantle can yield melts less silicic than andesitic compositions with < 57 wt% SiO2 (Lloyd et al., 1985; Baker et al., 1995). Alkaline basalts from Puyuan seamount have low SiO2 ((41.97 – 44.96) wt%), high contents of MgO ((3.37 – 5.16) wt%), Cr ((56 – 204)× 10−6), and Ni ((85 – 165)×10−6; Table 2), suggesting they might be derived from partial melting of mantle (Frey et al., 1978; Herzberg and Asimow, 2008). Some basalt samples with low MgO might have experienced fractionation of plagioclase, olivine and pyroxene. However, the fractional crystallization of plagioclase should be negligible because of no Eu negative anomaly (Eu/Eu* = 0.89 – 1.06). The chondrite-normalized REE patterns of all alkaline samples exhibit pronounced enrichment in LREE relative to HREE, which are in good accordance with those of OIBs (Fig.4b; Sun and McDonough, 1989). Their OIB affinity is further suggested by enrichment in radiogenic Sr-Nd-Pb isotopes and their 'Dupal isotopic anomaly' feature, roughly consistent with those of plume-related OIB-like basalts in the Indochina peninsula (Fig.5; Hoang et al., 1996, 2013; Wang et al., 2012b, 2013). In Fig.5a, the Sr-Nd isotopic compositions of Puyuan basalts define a binary mixing trend between the proposed depleted mantle (DM) and enriched mantle Ⅱ (EMII) end members. In detail, they originate from the DM mixing with 10% – 20% of EMII.

FC3MS value (FeO/CaO−3*MgO/SiO2, all in wt%) can be used to determine whether basalts are pyroxenite-derived melts or peridotite-derived melts even if fractionation process of basaltic magmas is taken into account (Yang and Zhou, 2013). Generally, the FC3MS values of experimental peridotite-derived melts are −0.07 ± 0.51 (2δ, n = 656), and the FC3MS values of experimental pyroxenite-derived melts are 0.46 ± 0.96 (2δ, n = 494), respectively. The FC3MS values of Puyuan basalts are 0.56 – 0.71, which is consistent with those of pyroxenite-derived melts rather than peridotite-derived melts (Fig.7a).

Fig. 7 (a–b) Plots of FC3MS vs. TiO2 and La/Yb ratio. Results of experiments (2δ) and batch melting models (blue and green curves) on pyroxenite and peridotite are based on Yang and Zhou (2013). (c) Plots of a La/Sm vs. Sm/Yb. The pyroxenite with a depleted MORB chemical composition (La of 1.5×10−6, Sm of 2.5×10−6, and Yb of 4×10−6) modeled with varying melting degrees (F) and mineral compositions (Cpx, clinopyroxene; Opx, orthopyroxene; Gt, garnet) are calculated by Zhang et al. (2018).

Furthermore, the batch melting model based on the FC3MS values and La/Yb ratios suggests that the Puyuan basaltic magmas could be generated by partial melting of average pyroxenite (Fig.7b). Additionally, low HREE contents (e.g., Er, Tm, Yb, Lu Y), steep chondrite-normalized REE patterns ([La/Yb]N = 6.75 – 16.27; Table 2) and high Er/Yb ratios (> 1; Table 2) suggest the origin of the Puyuan magmas is within the garnet stability field. In detail, garnet has higher partition coefficients for the element Yb (3.9) and Er (2.8) (http://www.earthref.org) and its fractionation from the parent magma would be expected to yield Er/Yb ratios of > 1 in the residual magma phase, thus either garnet fractionation process or partial melting of mantle with garnet in the residual would produce fractionated HREE (e.g., Er/Yb > 1) in the basaltic liquid phase. Based on the La/Sm and Sm/Yb ratios, quantitative modeling results suggest that the Puyuan basalts can be formed by low degree melting (< 1%) of the garnet-bearing pyroxenite (Fig.7c).

5.1.2 E-MORB type basalt from Beipo seamount

Tholeiitic basalts from Beipo seamount have E-MORB-like trace elemental characteristics (Fig.4). Absence of Eu negative anomaly (Eu/Eu* = 1.05 – 1.11) rule out the possibility of plagioclase fractional crystallization. We compare the isotopic compositions of the Beipo basalts with the plume-related basalts (Tu et al., 1992; Hoang et al., 1996, 2013; Yan et al., 2008; Han et al., 2009; Zou and Fan, 2010; Li et al., 2013; Expedition 349 Scientists, 2014), and find that the Hainan-Indochina-fossil ridge basalts that were derived from the Hainan mantle plume overlap with the counterparts from the Beipo seamount in Sr-Pb isotopic compositions. However, the Beipo basalts have higher εNd values than those of plume-related basalts, suggesting a more 'depleted' mantle origin than plume-like mantle. This is consistent with the less contribution of EMII (< 10%) in mantle source, which is inferred from the Sr-Nd isotopic compositions of the Beipo tholeiitic basalts (Fig.5). Thus, the Hainan mantle plume might have a minor role in the generation of the Beipo basalts.

The FC3MS values of the Beipo tholeiitic basalts are 0.69, which fall in the range of pyroxenite melts (Fig.7a). In Fig.7b, the FC3MS values and La/Yb ratios of Beipo basalts are close to the batch melting curve of average pyroxenite rather than that of peridotite. In fact, the La/Sm and Sm/Yb ratios of the tholeiitic basalts from Beipo seamount can be yielded by 3% – 10% melting of the pyroxenite without or with minor garnet (Fig.7c). These modeling results suggest that these E-MORB type basalts were probably formed by high degree melting of pyroxenite at shallow mantle depths (Zhang et al., 2018).

5.2 A Tectonic Evolution Scenario for Puyuan and Beipo Seamount Volcanism

Different types of basalts may originate by different tectonic processes at different times. The Puyuan seamount in the northeastern SCS is probably related to the Hainan mantle plume, which is evidenced by the following works: 1) according to the whole-rock 40Ar/39Ar ages (22 – 21 Myr; Wang et al., 2012a), Puyuan seamount was formed during the syn-spreading period of the SCS that has been proposed to be triggered by the Hainan mantle plume (Yan and Shi, 2007; Yan et al., 2014; Zhang et al., 2018; Yang et al., 2019); 2) the OIB-like Puyuan basalts have similar geochemical compositions to the plume-related basalts in the Indochina peninsula (Hoang et al., 1996, 2013; Wang et al., 2012b, 2013); 3) the Sr-Nd isotopic compositions of Puyuan basalts have indicated the high contribution (10% – 20%) of EMII that has been proposed as the main component of the Hainan mantle plume (Yan et al., 2008; Zou and Fan, 2010; Wang et al., 2012b). Furthermore, the Puyuan basalts originated from garnet-bearing pyroxenite source, probably reflecting an eclogite-rich plume that impacted on the SCS. The conclusion is consistent with the results evidenced by thermometric data from Hole U1431E of IODP Expedition 349 and 3D geodynamic modeling, which suggest that the Hainan mantle plume might contain recycled Indian and Pacific oceanic crust in the form of eclogite (Zhang and Li, 2018; Yang et al., 2019).

However, other mechanisms, rather than the Hainan mantle plume, might have important effect on the Beipo volcanic events, due to apparent transition of rock types from alkaline OIBs to tholeiitic E-MORBs and low EMII contribution in the Beipo mantle source (Figs. 3, 4 and 5). The Beipo seamount was inferred to be formed in the period of post-spreading (about 5.5 Myr) based on the seismic section data from the Dongsha Rise and the adjacent depressions in the northeastern SCS (Fig.1; Wu et al., 2014; Gao et al., 2015, 2016). Previous studies have shown that post-rifting movement with extensive uplift occurred at the northeastern margin of SCS after the cessation of the seafloor spreading (Lüdmann and Wong, 1999; Lüdmann et al., 2001; Wu et al., 2014).The post-rifting mechanism was inferred to be associated with subduction of the SCS slab beneath the Philippine Sea Plate at the Manila trench (McIntosh et al., 2013). In this model, large resistance stresses of the subducted crust with lower density may have led to lithosphere bending and post-rifting in the Dongsha and adjacent area (Wu et al., 2014). Melting of the lithospheric mantle beneath the lower slope of the northern margin might generate the Beipo seamount that represents E-MORB type volcanic rocks. Based on geochemical data, they were formed by relatively high degree melting of pyroxenite at shallow mantle depths, without or with minor garnet in the mantle source. Additionally, the relatively 'depleted' Sr-Nd isotopic compositions suggest that the enriched Hainan mantle plume have less effect on the post-spreading volcanism in Beipo seamount than the syn-spreading volcanism in Puyuan seamount (Fig.5a).

Collectively, alkaline OIB type basalts from the Puyuan seamount reveal an syn-spreading, eclogite-bearing Hainan mantle plume, whereas tholeiitic E-MORB type basalts in the Beipo seamount were derived by decompression melting of pyroxenite mantle without or with minor garnet, which was trigged by lithosphere bending and subsequent post-rifting at the lower slope of the northern margin.

6 Conclusions

Volcanic rocks from the Puyuan and Beipo seamounts within COT zone show different geochemical compositions, probably reflecting different petrogenesis and tectonic evolution stages from Miocene to Pliocene. Puyuan OIB-type volcanic rocks reveal an isotopically 'enriched', and garnet pyroxenite mantle transferred by the Hainan mantle plume, whereas E-MORB type rocks in the Beipo seamount were derived by decompression melting of isotopically 'depleted' pyroxenite mantle, which was trigged by lithosphere bending and post-rifting at the lower slope of the northern margin.

Acknowledgements

This study was jointly supported by the National Key R & D Program of China (No. 2018YFC0309802), the 13th Five-Year Plan Program of the China Ocean Mineral Resources Research and Development Association Research (No. DY135-S2-2-08), the Soft Science Project of Shandong Province Key Research and Development Plan (No. 2019 RZA02002), the China Postdoctoral Science Foundation (No. 2017M610403), and the Taishan Scholar Project Funding (No. tspd20161007).

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收稿日期：2019-05-09；修订日期：2019-11-21；接受日期：2020-01-13